Over a decade ago, we demonstrated that the latent viral reservoir in the resting CD4+ T cell compartment persists in virtually all HIV-infected individuals receiving clinically effective ART. Consequently, this viral reservoir is a major impediment to the eradication of HIV in infected individuals on ART. In addition, we demonstrated that HIV continually replicates at low levels in chronically infected individuals who are consistently aviremic during prolonged periods of receiving ART. During the past year, we have focused our research on: 1) delineating the mechanisms by which HIV persists in infected individuals receiving ART for extended periods of time, and 2) investigating the relationship between residual plasma viremia and the size of HIV reservoirs in infected individuals receiving ART. First, we examined the HIV burden in a subset of resting CD4+ T cells expressing Programmed Death (PD)-1 in infected individuals receiving ART. It has been proposed that the HIV present in infected resting CD4+ T cells is immunologically and virologically quiescent. We demonstrated that resting CD4+ T cells that express PD-1 carried substantially higher levels of HIV proviral DNA compared with PD-1-negative resting CD4+ T cells in the blood of HIV-infected individuals receiving effective ART (plasma viremia below 50 copies HIV RNA/ml). The majority of PD-1+ resting CD4+ T cells expressed CXCR3, a tissue-homing receptor, suggesting that these cells may have recently migrated out of various tissue sites into the peripheral blood. PD-1+ resting CD4+ T cells from the majority of patients receiving clinically successful ART spontaneously released HIV in the absence of any activating stimuli. The pattern of mRNA expression involving cellular activation and cytokine and chemokine genes in PD-1+ resting CD4+ T cells also was distinct from that of PD-1-negative resting and activated CD4+ T cells in the study participants. Taken together, our data suggest that PD-1+ resting CD4+ T cells represent a unique population of infected cells that persist during effective ART in HIV-infected individuals. Second, we investigated the relationship between residual plasma viremia and the size of HIV reservoirs in infected individuals receiving ART. In recent years, studies utilizing a laboratory-based real-time PCR assay have shown that residual plasma viremia below 50 copies/ml can be observed in certain infected individuals receiving ART. However, the relationship between the degree of residual plasma viremia, the size of HIV reservoirs, and the level of immune activation has not been delineated. We demonstrated detectable levels of plasma viremia (1-49 copies, median 2.7) in the majority of study participants receiving ART in whom plasma viremia had been suppressed for extended periods of time, as measured by a standard clinical assay (limit of detection of 50 copies/ml). The plasma from the majority of the study participants contained residual plasma viremia (63.3%) whereas there was no measurable HIV RNA in the plasma of 36.7% of the individuals we studied. There was a direct correlation between the level of residual plasma viremia and the frequency of CD4+ T cells carrying HIV proviral DNA (p=0.002). Of note, 38% of the study participants with undetectable plasma viremia carried greater than 766 copies (median of all data points) of HIV DNA per 106 CD4+ T cells, suggesting that at least in some individuals, the size of HIV reservoir in the CD4+ T cell compartment may not necessarily equate to the level of residual plasma HIV. With regard to the effect of immune activation on residual plasma viremia and the size of the viral reservoir in infected individuals receiving clinically effective ART, markers of immune activation (C-reactive protein, D-dimer, IL-6, soluble TNF receptor, and CD38 expression on CD4+ and CD8+ T cells) did not correlate with residual plasma viremia and the frequency of peripheral blood CD4+ T cells carrying HIV proviral DNA (p>0.5). Considering the relatively high frequency of HIV infection in the peripheral blood CD4+ T cell compartment of nearly 40% of the study participants exhibiting undetectable plasma viremia (0 copies/ml) and the lack of any correlation between residual plasma viremia and various markers of immune activation in blood, our data suggest that reactivation of the latent viral reservoir may not be the sole source of residual plasma viremia and persistently infected cellular reservoirs in various lymphoid tissues may also contribute to detectable HIV in plasma.